Conventionally, a plurality of cold-cathode tubes (CCFLs: Cold Cathode Fluorescent Lamps) are used for a backlight of a liquid crystal display panel in a liquid crystal television receiver (hereinafter referred to as the “liquid crystal TV”), a liquid crystal monitor, or the like (see, for example, Japanese Patent Application Laid-Open No. 2004-213994 (FIG. 1) (Patent document 1)).
FIG. 4 is a circuit diagram showing a conventional cold-cathode tube drive device. In the device shown in FIG. 4, an inverter circuit 101 generates a high-frequency voltage, a booster transformer 102 boosts the high-frequency voltage generated by the inverter circuit 101 to apply the high-frequency voltage after boosted to a cold-cathode tube 103. A resonant capacitor 104 is a capacitor composing, together with a secondary winding of the booster transformer 102, a resonant circuit.
The inverter circuit 101 acquires a drop voltage by a resistor 105 via a diode D and detects a conducting current value of the cold-cathode tube 103 based thereon to thereby control lamp current of the cold-cathode tube 103 in accordance with the value.
Based on the circuit configuration as described above, the lamp current of the cold-cathode tube 103 is detected directly from a lower voltage portion of the cold-cathode tube 103.
In a liquid crystal display panel of a medium-size or below, the cold-cathode tube 103 is relatively short, in which stray capacitance caused by two wirings 106a, 106b disposed to be connected to both the electrodes of the cold-cathode tube 103 has a smaller impact, so that one of the two wirings 106a, 106b connected to both the electrodes of the cold-cathode tube 103 is integrated into the other.
Meanwhile, in a liquid crystal display panel of a large size, namely a widescreen being developed recently, the cold-cathode tube 103 of a long size is used and thereby the two wirings 106a, 106b have a longer length. Therefore, when one of the two wirings 106a, 106b is integrated into the other, the stray capacitance increases to increase current leakage, in which luminance down and efficiency down in lamp wattage with respect to the output of the inverter circuit 101 are caused.
Therefore, when the long cold-cathode tube 103 is used, the one of the two wirings 106a, 106b connected to both the electrodes of the cold-cathode tube 103 is not integrated into the other, and, instead, the wiring 106b at the lower voltage side is directly connected to a case or the like in the vicinity of the root of the cold-cathode tube 103 as a ground. FIG. 6 is a circuit diagram showing a conventional cold-cathode tube drive device for a long cold-cathode tube. In addition, FIG. 7 is a circuit diagram showing a conventional cold-cathode tube drive device for a plurality of the long cold-cathode tubes.
In the above cases, the wiring 106b at the lower voltage side is directly connected to the ground in the vicinity of the cold-cathode tube 103, in which the lamp current value cannot be detected directly from the lower voltage side of the cold-cathode tube 103, so that the conducting current of the secondary winding of the booster transformer 102 is detected as a lamp current from the drop voltage by the resistor 111.
Patent document 1: Japanese Patent Application Laid-Open No. 2004-213994 (FIG. 1).